Field of the Invention
The present invention relates to multilayer diffractive optical elements. In particular, the present invention relates to a multilayer diffractive optical element in which two layers of resin each having a diffraction grating pattern are interposed between substrates of glass or the like.
Description of the Related Art
A multilayer diffractive optical element having a diffraction grating pattern formed on a boundary face between two kinds of optical materials is known as a diffractive optical element to be used for a lens or the like. Japanese Patent Laid-Open No. 2007-212547 discloses the following technique. Specifically, a resin layer having a diffraction grating pattern is formed on a glass substrate, and then a resin having different optical characteristics is interposed between the formed resin layer and another glass substrate and is cured to thus be bonded.
In addition, Japanese Patent Laid-Open No. 2013-160773 discloses a method for manufacturing a diffractive optical element having a grating pattern with high precision. In that method, the diffractive optical element is manufactured by using a mold having a groove portion provided at a periphery of the grating pattern.
However, the multilayer diffractive optical element manufactured through the manufacturing method disclosed in Japanese Patent Laid-Open No. 2013-160773 has shortcomings in that resin peeling occurs after the diffractive optical element is used for an extended period of time in a high-temperature environment.
With regard to what causes the resin peeling to occur, the present inventor has found the following. When a diffractive optical element is manufactured through the manufacturing method disclosed in Japanese Patent Laid-Open No. 2013-160773, a diffractive optical element having a protrusion at a periphery of a diffraction grating pattern is obtained. When a multilayer diffractive optical element is manufactured with the use of the aforementioned diffractive optical element through the manufacturing method disclosed in Japanese Patent Laid-Open No. 2007-212547, uncured resin is placed on the diffractive optical element. Since there is a step due to the protrusion at the periphery of the diffractive optical element, when the uncured resin is cured, large stress remains at the step portion due to the cure shrinkage. In addition, when a multilayer diffractive optical element is used for an extended period of time, the bonding strength at an interface between a resin layer and a glass substrate gradually decreases due to the moisture in the air entering therebetween. Furthermore, when a multilayer optical element is used for an extended period of time, the bonding strength at the interface becomes smaller than the stress remaining at the step portion, and thus the resin peeling occur in some cases.
In the meantime, in the field of adhesive technology, a technique is known in which a portion with a step is made into a gently sloped shape or a tapered shape so as to allow the film thickness of the adhesive to vary continuously. Thus, the stress that remains when the adhesive is cured is dispersed so as to prevent the peeling from occurring.
However, when the tapered shape is applied to the multilayer diffractive optical element, a broad area needs to be secured outside an optically effective region in order to sufficiently disperse the stress to an extent that the resin peeling is prevented from occurring, and there is a problem in that the size of the optical element is increased.